Is it best to just ask for whoever is available that day, pricing wise. If I ask for a specific provider, does that ensure the price goes up? Thanks for all your help with my rather obvious questions.

you should never assume that all of the ladies working for an agency work on the same price structure.

and price should not be an issue, nor should upselling.  One reason you should carefully read reviews and do proper searches on TER.  To get a good idea of the caliber of an agency, go to the search reviews section and put in their phone number- you should get a list of providers who work for that agency... read the reviews and make a determination for yourself.
There ARE extremely reputable agencies out there.  
My personal preference is to schedule with indies and avoid agencies that are not well known in my area.

Ask for whoever is available, I prefer to know who is going to show up at the door. But that should not up the price, but not all ladies at an agency are necessarily at the same price.